OF THE FOETUS. 305 



lition of which is invariable. The back-bone 

 is the foundation of the animal frame ; and the 

 action and movements of moft members of the 

 body depend upon the fpinal marrow which that 

 bone contains. It is this part, alfo, together with 

 the head, which appears lirft in the embryo. 

 Now, thcfe fimple parts, which are firO: formed, 

 are all efTential to, the exiftence and the form of 

 the animal. 



The double parts in an animal body are more 

 numerous than the fingle parts; and they feem 

 to be produced on each fide of the fingle parts by 

 a fpecies of vegetation; for the double parts arc 

 fimilar in form, and different only in pofiiion. 

 The left hand is perfedly fimilar to. the right ; 

 but, if the left hand w^ere placed in the fituatioii 

 of the right, we could not perform the fame ac- 

 tions with it. The fame thing may be obferved 

 of all the double parts : They are fin:iilar in 

 form, but differ in pofition, which is relative to 

 the body of the particular animal ; and, if a line 

 were drawn, dividing the body into two equal 

 parts, the poiition of all the fimilar parts would 

 tend to tliis line as to a centre. 



I he fpinal marrow, and the vertebrae in 

 which it is inclolcd, appear to be the real axis 

 of all the double parts of the animal body, from 

 which they feeni to derive their origin, and to 

 be only proportional branches iffuing from this 

 trunk or common bafe; for, in the young chick, 

 we fee the ribs fliooting from each fide of the 



Vol. If. U vertebrae, 



